As semiconductor integrated circuits become microminiaturized and more highly integrated, shortening the wavelength of an exposure light source that is used in manufacturing the circuits is necessary, and an extreme ultraviolet light source apparatus (hereinafter also referred to as an EUV light source apparatus), which emits extreme ultraviolet light (hereinafter referred to as EUV (Extreme Ultra Violet) light) having a wavelength of 13-14 nm, especially a wavelength of 13.5 nm, has been developed (for example, refer to Patent Document 1) as a next generation exposure light source for semiconductor.
FIG. 4 is a diagram for simplified explanation of the EUV light source apparatus disclosed in the PATENT LITERATURE 1.
As shown in the figure, the EUV light source apparatus has a chamber 1, which is an electric discharge container. In the chamber 1, an electric discharge part 1a, in which a pair of disc-shaped electric discharge electrodes 2a and 2b is accommodated, and an EUV light focusing part 1b, in which a foil trap 5 and a collector mirror 6 are accommodated.
The pair of disc-shaped electrodes 2a and 2b is arranged in upper and lower parts as shown in FIG. 4, with an insulating member 2c interposed therebetween. A rotation axis 2e of a motor 2j is attached to the electric discharge electrode 2b, which is located at a lower portion as shown in FIG. 4.
The electric discharge electrodes 2a and 2b are connected to a pulse power supplying unit 3 through sliding elements 2g and 2h. A groove portion 2d is formed in a peripheral portion of the electric discharge electrode 2b, and solid material M (Li or Sn) for generating a high temperature plasma P is arranged in this groove portion 2d. 
A gas discharge unit, which discharges gas from the electric discharge part 1a and the EUV light focusing part 1b, thereby forming a vacuum stage in the inside of the chamber 1 is referred to as 1c. 
In the EUV light source apparatus, the material for a high temperature plasma, which is arranged at the groove portion of the electric discharge electrode 2b, is irradiated with an energy beam from an energy beam radiation device 4. The energy beam is, for example, a laser beam, and is emitted therefrom through the laser incident window 4a, and the solid material evaporates between the electric discharge electrodes 2a and 2b. 
In this state, when pulse power is supplied between the electric discharge electrodes 2a and 2b from the pulse power supplying unit 3, electric discharge occurs between an edge part of the electric discharge electrode 2a and an edge part of the electric discharge electrode 2b, so that a plasma P is formed due to the high temperature plasma material M, whereby it is heated and excited by large current, which flows at time of electric discharge, and EUV light is emitted from this high temperature plasma P.
The emitted EUV light enters the EUV light focusing part 1b through a foil trap 5, is focused at a middle focal point f of a collector mirror 6 by the collector mirror 6, is emitted from an EUV light emitting mouth 7, and enters an exposure device 40 shown by dotted lines, which is connected to the EUV light source apparatus.